With regard to the paint used as a resin for precoated metal sheets, there are many paints including copolymerized polyester resin, alkyd resin, vinyl resin and silicon resin paints and they are used in a form of a solvent system, aqueous dispersion system or powder paint. However, in any of those cases, there is no product which satisfies all of the requested properties and it is unavoidable that, when one property is given the top priority, other properties will become unsatisfactory whereby it is the real fact that there is a limit upon actual use.
Moreover, there has been a demand of drawing processability in recent years not only for two-piece food/beverage cans but also for precoated metal sheets because of giving high added-value. In the meanwhile, clears, color clears and metallic paints with high ornamental value as well as deep color paints with low pigment concentrations have been investigated. However, any of them does not have sufficient property at present whereby various problems have been left unsolved or their practical utilization has been delayed.
For example, in the Japanese Kokai Patents Sho-57/57,746 and Sho-63/108,081, investigations were made for giving both hardness and processability but sufficient effects are not achieved in any of them. In the Japanese Kokoku Patent Sho-62/21,830, coating resin compositions mainly comprising terephthalic acid and alkylene glycol were investigated but their gloss retention (GR) is poor upon drawing processing and, therefore, they have not been actually used. In turn, the product showing a relatively good processability has insufficient hardness and, especially in the case of clear or low-pigment products, hardness, scuff resistance and stain resistance are considerably insufficient. Further, alkali resistance, weather resistance and gasket resistance are insufficient too.
Another coating resin composition mainly comprising terephthalic acid and alkylene glycol has been also known from the Japanese Kokoku Patent Sho-61/34,754. Again, its drawing processability is insufficient and, in the case of clear or low-pigment products, hardness, scuff resistance and stain resistance are considerably insufficient.
In the Japanese Kokai Patent Hei-07/18,169, a coating resin composition mainly comprising isophthalic acid, orthophthalic acid and long-chain glycol is disclosed and, although the resulting product exhibits good weather resistance, alkali resistance and processability, its drawing processability is insufficient and its hardness, stain resistance and scuff resistance are poor.
In said Japanese Kokai Patent Hei-07/18,169, a coating resin composition mainly comprising isophthalic acid, orthophthalic acid and alkylene glycol is investigated as well and, although the reuslting product exhibits good processability, weather resistance and alkali resistance, its hardness, blocking resistance pressure-mark resistance and stain resistance are considerably insufficient. Further, in spite of its low glass transition temperature (Tg), its drawing processability is insufficient.
In the Japanese Kokai Patent Sho-59/91,118, polyester in which 2-methyl-1,3-propanediol is used is known. Its object is to reduce the cost and to improve the workability upon polymerization by the use of 2-methyl-1,3-propanediol in place of neopentyl glycol and the product is a polyester having a lower molecular weight wherein a large amount of tri- or higher functional polyols are jointly used. Therefore, large amount of aliphatic dicarboxylic acid is copolymerized for achieving a processability but it does not give a sufficient result but processability, stain resistance, weather resistance, etc. are poor whereby the product is entirely unsuitable for coating resins.
In the polyester resins for precoated metal sheets, drawing processability is insufficient and there is a problem of reduction of gloss in the processed area even in the case of polyesters of low Tg having a good processability. Thus, if one wants to prepare clear or low-pigment concentrations having good hardness, it is usually necessary to make Tg not lower than 40.degree. C. whereby the coat becomes considerably hard and good processability and impact resistance are not achieved. On the other hand, if one wants to achieve good processability and impact resistance, hardness will be reduced whereby good scuff resistance is not afforded and, in addition, problems such as blocking and pressure mark are resulted.